From the viewpoint of energy problems and environmental problems, CO2 is required to be efficiently reduced using light energy, such as in plants. Plants use a system called the Z-scheme which excites light energy in two stages. Due to a photochemical reaction in such a system, plants oxidize water (H2O) to obtain electrons, and, thus, to reduce carbon dioxide (CO2), thereby synthesizing cellulose and sugars.
However, the technologies for obtaining electrons from water, via dissolving CO2 through an artificial photochemical reaction without using any sacrificial reagent, are of very low efficiency.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2011-094194 as a photochemical reactor is provided with an electrode for an oxidation reaction for oxidizing H2O to produce oxygen (O2) and an electrode for a reduction reaction for reducing CO2 to produce a carbon compound. The electrode for oxidation reaction is provided with an oxidation catalyst for oxidizing H2O on a surface of a photocatalyst and gains a potential via the light energy. The electrode for reduction reaction is provided with a reduction catalyst for reducing CO2 on a surface of the photocatalyst and is connected to the electrode for oxidation reaction with an electric wire. The electrode for reduction reaction gains a reduction potential of CO2 from the electrode for oxidation reaction to reduce CO2, and, thus, to produce formic acid (HCOOH). Thus, to gain the potential necessary for reducing CO2 using an optical wavelength and a photocatalyst, the photochemical reaction device employs a Z-scheme-type artificial photosynthesis system that imitates plants.
However, in Jpn. Pat. Appln. KOKAI Publication No. 2011-094194, the solar energy conversion efficiency is very low at around 0.04%. This is because the energy efficiency of the photocatalyst excited by the optical wavelength is low. Since the electrode for reduction reaction is connected to the electrode for oxidation reaction with an electric wire, the efficiency in extracting electricity (electric current) decreases due to interconnection resistance, thus the efficiency becomes low.
S. Y. Reece, et al., Science. vol. 334. pp. 645 (2011) provides a configuration in which a silicon solar cell is used for achieving the reaction potential and catalysts are provided on both sides of the silicon solar cell to produce a reaction. The solar energy conversion efficiency is very high at around 2.5%. Since this device has a structure requiring no wiring, it can be easily increased in size. As examples of the features of the device, a cell itself serves as a divider plate to allow for isolation of a product, and, thus, to eliminate a process for separating the product.
However, for this device, there are no examples of success in the reduction reaction of CO2. Such a plate-like laminated structure does not take into consideration the fact that, for the CO2 reduction reaction, ions with a positive charge generated on the oxidation side and ions with a negative charge generated on the reduction side need to move to the opposite sides. In an oxidation-reduction reaction in which H2O is used as an electron donor instead of a sacrificial catalyst, in particular, proton (hydrogen ion (H+)) movement or hydroxide ion (OH−) movement is indispensable.
Y. Hori, et al., Electrochim. Acta. 1994, 39, 1833 reports CO2 reduction activities in various metal electrodes. In a CO2 reduction reaction, electrons and protons are reacted with CO2 to produce a hydrocarbon such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH), and methane (CH4). Hydrocarbons produced by a reduction reaction differ depending on the number, of electrons obtained by the reduction reaction. For example, carbon monoxide and formic acid are produced by a reaction with two electrons, methanol is produced by a reaction with six electrodes, and methane is produced by a reaction with eight electrons. In a reaction for producing any hydrocarbon, a standard oxidation-reduction potential is substantially equivalent to a reaction for reducing hydrogen ions to produce hydrogen. However, a large overvoltage (excess energy) is actually required for a reduction reaction of a first electron, and the reduction reaction hardly proceeds. In a reduction reaction with a larger number of electrons, it is more difficult to advance the reduction reaction with a Faraday efficiency. In order to carry out a CO2 reduction reaction for producing a desired hydrocarbon with selectively high efficiency, a highly active electrode catalyst is required.
In Yu Sun, et al., Chem. Lett. 2012, 41, 328, a CO2 reduction reaction is carried out in an electrode provided with an organic molecule for solidifying an Au catalyst on an Si substrate. More specifically, the Si substrate absorbs light to separate charges, and, thus, to produce an electron. The produced electron is then transferred to the Au catalyst through a molecule bound to the Si substrate, and the CO2 reduction reaction is carried out on the Au catalyst.
As described above, a CO2 reduction technique having high reaction efficiency is required.